Some internal combustion engines mounted on vehicles have an O.sub.2 sensor disposed as an exhaust sensor in an exhaust passage and are provided with a control means which makes a feedback control in accordance with a detection signal output from the O.sub.2 sensor so that the air-fuel ratio becomes a target value.
Such a method and apparatus for controlling the air-fuel ratio in an internal combustion engine are disclosed in Japanese Patent No. 2524359. According to a fuel controller for an internal combustion engine disclosed in this patent, a flow characteristic correction quantity in a fuel supply means, an output characteristic correction quantity in an intake air volume detecting means and an invalid time correction quantity in the fuel supply means are separated from a learning value of a correction quantity related to the amount of fuel supplied and are updated while selecting a load region in which the accuracy of the correction quantities is improved, thereby performing an air-fuel ratio feedback control in high response and controlling the fuel supply in an open control with a high accuracy.
According to a learning control method for the airfuel ratio in an internal combustion engine disclosed in Japanese Patent Publication No. 6-35850, when a shift is made from a state in which a feedback control is not executed to a state in which a feedback control is being executed, learning is prohibited by a predetermined skip count.
According to an air-fuel ratio learning control apparatus disclosed in Japanese Patent Publication No. 7-51907, even when an operating condition of an engine is in the vicinity of a boundary portion of an operation zone which is set in one memory means, if in an operation zone set in the other memory means the engine operating condition does not lie in the boundary portion and is a steady operating condition, learning is always performed by one of the memory means.
In Japanese Patent Laid Open No. 8-261043 there is disclosed a learning control method for the air-fuel ratio in an internal combustion engine in which a basic fuel injection volume is calculated on the basis of both the opening of a throttle valve disposed in an intake system of the engine and the number of revolutions of the engine, then a feedback correction quantity is calculated with a predetermined period and in accordance with an output signal provided from an O.sub.2 sensor mounted in an exhaust system, then the basic fuel injection volume is corrected on the basis of at least the feedback correction quantity to determine a final fuel injection volume, and a learning control is made for the air-fuel ratio. According to this learning control method, when a predetermined time has elapsed until reversal of the output signal, an auxiliary correction quantity is calculated from both a feedback correction quantity calculated before lapse of the predetermined time and a feedback correction quantity of this time calculated upon lapse of the predetermined time, then a learning correction quantity is calculated on the basis of the thus-calculated auxiliary correction quantity, and where the thus-calculated learning correction quantity satisfies predetermined conditions, the learning correction quantity stored in the learning zone concerned is updated quickly using the calculated learning correction quantity.
In Japanese Patent Laid Open No. 42025/97 there is disclosed a control apparatus for controlling the air-fuel ratio in an internal combustion engine, comprising a fuel injection valve which injects a fuel fed under pressure from a fuel tank into a combustion chamber in the internal combustion engine, an air-fuel ratio detecting means disposed in an exhaust system of the engine to detect an air-fuel ratio from exhaust gases, an air-fuel ratio correction coefficient calculating means for calculating an air-fuel ratio correction coefficient in accordance with the detected air-fuel ratio, a feedback control means for feedback-controlling an operation quantity of the fuel injection valve on the basis of the air-fuel ratio correction coefficient which is calculated in such a manner that the air-fuel ratio falls under a predetermined range, a learning control means which learns an air-fuel ratio correction quantity according to an operating condition of the engine while changing an update quantity according to a fetch count or fetch time of the air-fuel ratio correction coefficient, and a correction means for correction the operation quantity of the feedback-controlled fuel injection valve in accordance with the learned air-fuel ratio correction quantity. According to this air-fuel ratio control apparatus, the change of updating the learning value is increased and there is realized an air-fuel ratio control of high accuracy.
In the conventional air-fuel ratio controlling apparatus for an internal combustion engine, the air-fuel ratio is feedback-controlled in accordance with a detection signal provided from an O.sub.2 sensor as an exhaust sensor and a learning correction of the air-fuel ratio is performed for absorbing variations in the internal combustion engine, sensors and various devices.
In the learning correction according to the prior art, as shown in FIG. 9 for example, a map based on the relation between engine load and engine revolution is divided into a plurality of zones (for example, sixteen zones from ZONE 1 to ZONE 16), and when an operating condition of the internal combustion engine has entered any of the zones, if the operating condition is a steady condition and if the skip of the feedback control has been conducted a preset number of times, the correction value as a learning value in the zone concerned is updated.
The above update control will now be described with reference to a prior art air-fuel ratio controlling flowchart of FIG. 10. Once an air-fuel ratio control program starts (300), judgment is made as to whether a feedback control is being executed or not (302), and if the answer is negative, the judgment is repeated until the answer becomes affirmative. If the answer in the judgment (302) is affirmative, the flow shifts to judgment (304) as to whether engine water temperature and intake air temperature conditions exist or not on the basis of detection signals provided from a water temperature sensor and an intake air temperature sensor, respectively.
If the answer in the judgment (304) is negative, the flow returns to the judgment (302) as to whether the feedback control is being executed or not, while if the answer in the judgment (304) is affirmative, the flow shifts to judgment (306) as to whether the engine operating condition is within a learning zone or not in such a map based on the relation between engine load and engine revolution as shown in FIG. 9.
If the answer in the judgment (306) is negative, the flow returns to the judgment (302) as to whether a feedback control is being executed or not, while if the answer in the judgment (306) is affirmative, there is made judgment (308) as to whether the operating condition is a steady condition, or a steady running condition, or not and if the answer is negative, the flow returns to the judgment (302) as to whether a feedback control is being executed or not, while if the answer in the judgment (308) is affirmative, the flow shifts to judgment (310) as to whether skip was executed or not in the feedback control after the operating condition had been judged to be a steady condition.
If the answer in the judgment (310) is negative, the flow returns to the judgment (302) as to whether a feedback control is being executed or not, and if the answer is affirmative, a counter is incremented (312).
After the counter incrementing process (312), there is made judgment (314) as to whether the count value of the counter has reached a preset count, i.e., a preset value, or more and if the answer is negative, the flow returns to the judgment (310) as to whether skip has been executed or not in the feedback control, while if the answer in the judgment (314) is affirmative, updating of a learning value, i.e., a correction value, is started (316), and after updating of the correction value, the flow shifts to Return (318).
The reason why skip is waited for by a preset count in the learning value or correction value updating learning control as noted above is that at the time of shift from an accelerating or decelerating condition to a steady condition there usually occurs a discrepancy in the air-fuel ratio under the influence of an increase or decrease of fuel corrected at the time of acceleration or deceleration.
The operation zone, when observed in detail, can be broadly divided into a zone (a steady running zone in FIG. 11) which is used mainly in a steady running at a constant speed, a zone (an acceleration zone in FIG. 11) which is used mainly in acceleration, and a zone (a deceleration zone in FIG. 11) which is used mainly in deceleration.
At present, however, even when the operating condition has shifted to any of the above three zones, the skip wait count until start of the updating learning control is constant.
As a result, there are formed a zone in which mislearning is apt to occur such as the acceleration zone and the deceleration zone and a zone (steady running zone) in which the correction value as a learning value is difficult to update although the occurrence of mislearning is less possible. Thus, it is difficult to obtain an exact correction value as a learning value and this point is one cause of discharge of exhaust gases containing harmful components.